The O, P, and Rainbow classes combined destroyer-sized displacement with a surfaced endurance of 10,000 nautical miles at 8 knots. Their eight-tube layouts supported a heavy opening salvo against a fleet, with reloads available for later opportunities or self-defense rather than an underwater chase. The fall of France removed French fleet and air support, forcing Britain to fill an immediate Mediterranean gap with boats intended for ocean service. Their size and surface-oriented operating pattern then made them more vulnerable in confined waters.
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O, P, and Rainbow classes: oceanic submarines
Large boats of the O, P, and Rainbow classes were designed for Pacific-range patrols rather than the confined and dangerous Mediterranean.